Effects of continuous light exposure on antioxidant enzymes, porphyric enzymes and cellular damage in the Harderian gland of the Syrian hamster Introduction The Harderian glands (HGs) are large orbital lachrymal glands present in most terrestrial vertebrates. In rodents, these tubulo-alveolar structures are particularly well devel- oped, especially in the Syrian hamster (Mesocricetus aura- tus) where they show a strong sexual dimorphism: the glands from female hamsters contain very high concentra- tions of porphyrins and a single secretory cell type (type-I), which is characterized by minute lipid droplets; however, glands from the male hamster have two secretory cell types, i.e. type-I, similar but not identical to the female type, and type-II cells, which are filled with large lipid vacuoles and contain smaller concentrations of porphyrins [1, 2]. The enzymes of the heme biosynthetic pathway exhibit much higher activities in the female than in the male HGs [3]. In the HG, 5-aminolevulinate synthase (ALA-S) activity is maximally expressed, and, because of the low ferroche- latase activity, there is an enormous amount of porphyrin [4–6]. Therefore, the HG contains an extremely high level of protoporphyrin, whereas heme is practically undetectable. There are several studies that show the inverse relationship between ALA-S mRNA and melatonin [7, 8]. The extraordinary rates of porphyrinogenesis make the HGs particularly interesting for studying damage by porphyrins and their precursors, which are common to the porphyrias. In these diseases, hormonally induced or congenital changes of heme biosynthesis cause accumula- tions of 5-aminolevulinic acid (ALA) and porphyrins in various tissues leading to porphyric attacks [9, 10]. Accumulation of porphyrins may produce free radicals after exposure to light [11–14]. Porphyrins cause toxic effects mediated by light, as porphyrins when absorbing ultraviolet or blue light (maximum absorption: 405 nm) form reactive excited electron states, implying that the HG, by its orbital location and accessibility to light would be subject to porphyrin phototoxicity. Apart from porphyrins, another source of reactive oxygen species in the HG is the high concentration of the precursor ALA which in the presence of light forms free radicals from its enolized subform [15, 16]. In most mammalian species, circadian rhythms are synchronized by the light/dark cycle via entrainment of the suprachiasmatic nucleus (SCN) representing the circa- dian pacemaker [17]. Because the SCN is neurally connec- ted to the pineal gland, the light/dark cycle also regulates Abstract: The Syrian hamster Harderian gland (HG), an organ present in the male two secretory cell types (type-I and type-II cells), is physiologically exposed to high oxidative stress because of high concentrations of porphyrins and their precursor, 5-aminolevulinic acid. Because of its juxtaorbital location, the HG is accessible to light, and subject to phototoxic effects of these substances. After having previously demonstrated circadian rhythms in antioxidant enzymes, porphyric enzymes and oxidative damage of proteins and lipids, as well as influences of melatonin on these parameters, we have now studied the effects of continuous light (LL), which suppresses melatonin secretion by the pineal gland. Measurements were performed in two different circadian phases, in order to detect the presence or absence of day/night differences. In LL, no differences between circadian phases of subjective day and subjective night were demonstrable for 5-aminolevulinate synthase, 5-aminolevulinate dehydratase, porphobilinogen deaminase, or superoxide dismutase; temporal differences in glutathione reductase and catalase were markedly diminished, whereas all these parameters showed marked day/ night differences in the rats exposed to a light/dark cycle of 14:10. In LL, oxidative damage to lipids was minimally effected, while protein damage was enhanced. LL also caused a reduction in the percentage of type-II cells. Therefore, cell differentiation in the HG does not seem to be controlled only by the androgen, but, unexpectedly, also by melatonin. Cristina Toma ´ s-Zapico 1 , Ana Coto-Montes 1 , Jorge Martı ´nez-Fraga, Marı ´a Josefa Rodrı ´guez-Colunga and Delio Tolivia Departamento de Morfologı ´a y Biologı ´a Celular, Facultad de Medicina, Universidad de Oviedo, Oviedo, Spain Key words: antioxidant enzymes, continuous light, Harderian gland, melatonin, porphyrogenesis Address reprint requests to Ana Coto-Montes, Departamento de Morfologı ´a y Biologı ´a Celu- lar, Facultad de Medicina, Universidad de Oviedo, C/ Julia ` n Claverı ´a, E-33006 Oviedo, Asturias, Spain E-mail: acoto@correo.uniovi.es 1 Both authors have contributed equally to this investigation. Received March 7, 2002; accepted September 9, 2002. J. Pineal Res. 2003; 34:60–68 Copyright Ó Blackwell Munksgaard, 2003 Journal of Pineal Research ISSN 0742-3098 60